In the past, such methods have not received much attention from developers of video equipment, although they have various useful applications. For example, such a quality evaluation may be employed in order to detect disturbances in a transmission channel for video data or for detecting defective scans of individual frames in a video stream generated by scanning analog video material. If a bad frame is detected in a video stream, the quality of the stream may be improved e.g. by discarding the bad frame, by retransmitting or rescanning it, or, if retransmitting or rescanning is unsuccessful, by restoring the bad frame based on e.g. frames preceding or succeeding it.